Acid

An acid is a water-soluble compound with a pH of less than 7. It is an ion or molecule that can give up a proton to a base, or accept an unshared pair of electrons from such a base. In a reaction with an alkali, the two compounds will create water and a salt. This salt is different with different combinations of acid and alkali.

Leonard McCoy and his mirror self both caused spilled acid on tables in their respective sickbays in 2266, creating a duplicate burn mark on each of them. McCoy noted the damage in 2267 as one thing which hadn't changed, as opposed to the rest of the mirror sickbay, which was disorganized and arranged differently than he left it. (TOS: "Mirror, Mirror")